Compiled by Stillwater Sciences.
Federal Endangered (Southern California steelhead distinct population segment; initial listing August 18, 1997 [NMFS 1997], final listing January 5, 2006 [NMFS 2006], recovery plan outline September 2007 [NMFS 2007])
Taxonomy and nomenclature
Oncorhynchus mykiss is one of several related Oncorhynchus species that exhibit considerable life history plasticity, namely the ability to complete their life cycle entirely in freshwater or migrate to the ocean as juvenile “smolts” and return to spawn in freshwater as adults after 1-3 years at sea (Boughton et al 2006). The freshwater resident form is commonly termed “rainbow trout”; the sea-going or anadromous form is typically referred to as “steelhead”. Adding to the complexity of O. mykiss life history is the apparent ability of rainbow trout to produce steelhead offspring (an anecdotally common occurrence in populations within the Santa Clara River watershed), and for steelhead to produce resident rainbow trout offspring. Further discussion of steelhead life history can be found below, and in Boughton et al (2006). This summary generally pertains to the anadromous form of O. mykiss within the southern California distinct population segment (NMFS 2006).
Steelhead occur throughout the North Pacific Ocean and historically spawned in freshwater streams along the west coast of North America from Alaska to northern Baja California. Historically, O. mykiss occurred at least as far south as Rio del Presidio in Mexico, although spawning populations of steelhead did not likely occur that far south (NMFS 1997). At present, the southernmost stream used by steelhead for spawning is generally considered to be Malibu Creek, California (NMFS 1997); however, in years of substantial rainfall, spawning steelhead may be found as far south as the Santa Margarita River, in northern San Diego County (NMFS 1997).
Historically (before circa 1946), steelhead likely spawned and reared in the major tributaries within the lower portion of the Santa Clara River system, west of the Piru Creek confluence (Kelley 2004, Harrison et al. 2006). These major tributaries included primarily Sespe and Piru creeks; Santa Paula and Hopper creeks likely provided significant steelhead habitat as well. A number of other tributaries in the upper (eastern) Santa Clara River system may have been used during wet years (Titus et. al., in preparation), though published information supporting their use by steelhead is generally lacking.
The present-day distribution of anadromous O. mykiss in the Santa Clara River watershed is modified by a number of complete and partial migration barriers that restrict upstream passage of adult steelhead, both in the lower mainstem river and most major tributaries. The Vern Freeman Diversion, approximately 16 km (10 mi) upstream from the mouth on the mainstem river, likely represents a partial barrier to upstream migration by returning adult steelhead; between 1994 and 1996 a total of four adult steelhead have navigated the fish ladder at the diversion (Entrix 2000). Passage within Santa Paula Creek, located approximately 8 km (5 mi) upstream of the Vern Freeman Diversion is limited by fish passage facilities damaged in the record 2005 floods . Upstream migratory access to Piru Creek, approximately 43 km (27 mi) upstream of Vern Freeman Diversion, was eliminated by the completion of Santa Felicia Dam in 1955. Sespe Creek, located approximately 28 km (17 mi) upstream of the Vern Freeman Diverson, is the only major steelhead spawning tributary to the Santa Clara River watershed that remains unregulated and accessible to upstream migrants (Titus et. al., in preparation).
The National Marine Fisheries Service (NMFS) has concluded that populations of naturally reproducing steelhead have been experiencing a long-term decline in abundance throughout their range (NMFS 1996a). Populations in the southern portion of the range have experienced the most severe declines (NMFS 1996a); NMFS estimates that the current southern steelhead population represents less than 1 percent of its historical population size (as cited in Stoecker 2002).
Prior to 1940, the Santa Clara River watershed is thought to have supported an average annual run of approximately 7,000-9,000 steelhead (Titus et. al., in preparation). Steelhead runs in the Santa Clara River may have been one of the largest in southern California (Stoecker and Kelley 2005, Titus et. al., in preparation). Recent counts at fish passage facilities associated with the Vern Freeman Diversion Dam indicate approximately 14 adult steelhead have returned to spawn in the Santa Clara River watershed since 1990 (Stoecker and Kelley 2005, Titus et. al., in preparation).
Southern steelhead in the Santa Clara River watershed have declined steeply since the 1950’s, mainly because of an increase in surface water diversion in the lower Santa Clara River (Titus et. al., in preparation). Other causes include diversions along the Santa Clara River, such as the diversion near Saticoy and the Vern Freeman Diversion Dam (Titus et. al., in preparation). Early CDFG records indicate that 5,000 juvenile steelhead were stocked in 1938, and 21,600 were planted in the lagoon at the mouth of the Santa Clara River in 1944 (Titus et. al., in preparation). Most of the fish planted in the lagoon were rescued from the Santa Ynez River (Titus et. al., in preparation). A two year study completed in 1985 yielded less than 30 steelhead adults in the lower Santa Clara River, and no emigrating smolts (Titus et. al., in preparation). The study concluded that the lower Santa Clara River served primarily as a migration corridor for adult and juvenile steelhead, while the estuary may still provide potential rearing habitat (Puckett and Villa 1985 as cited in Titus et. al., in preparation).
Life History and Timing
Steelhead is the term used to distinguish anadromous populations of O. mykiss from resident populations. Much life history variability exists among steelhead populations; however, populations may be broadly categorized into two reproductive groups, most commonly referred to as either winter-run or summer-run. South of San Francisco Bay, all steelhead are all winter-run. In the Santa Clara River watershed, the O. mykiss population appears to consist primarily of resident fish, possibly due to partial or complete migration barriers (both natural and anthropogenic) that preclude anadromous adults from reaching spawning tributaries. However, small numbers of anadromous juvenile steelhead (smolts) outmigrate from the Santa Clara River each year, presumably produced by the existing resident adult population. The relationship between anadromous and resident life history forms of this species is the subject of ongoing research. Current evidence suggests that either life history form can produce offspring that exhibit the alternate form (i.e., resident rainbow trout can produce anadromous progeny and vice versa) (Shapovalov and Taft 1954, Burgner et al. 1992, Hallock 1989). The fact that little to no genetic differentiation has been found between resident and anadromous life history forms inhabiting the same basin supports this hypothesis (Busby et al. 1993, Nielsen 1994, but see Zimmerman and Reeves 2000).The life history patterns of southern California steelhead depend more strongly on rainfall and flow than steelhead populations found farther north (NMFS 1997, Titus et al. in press). In southern California, average rainfall is substantially lower and more variable than in regions to the north, resulting in increased duration of sand berms across the mouths of streams and rivers and, in some cases, complete dewatering of the lower reaches of these streams from late spring through fall (NMFS 1997, Entrix 2002). Steelhead in southern California appear to withstand higher temperatures than populations to the north (NMFS 1997). Although there is minimal life history information for southern California steelhead, several unique traits have been identified, including increased temperature tolerance, duration and timing of life stages, and environmental flexibility (Stoecker and Kelley 2005, Titus et. al., in press).
Adult Upstream Migration and Spawning
Adult steelhead return to spawn in their natal stream, usually in their fourth or fifth year of life (Shapovalov and Taft 1954, Behnke 1992). Access to natal streams is often impaired or blocked because of low flow conditions (Stoecker 2002). Southern steelhead time their upstream migration to follow sizable rainfall events in the fall (Stoecker 2002). A unique adaptation of southern steelhead is the ability to delay the upstream migration until adequate flows exist, or ascend another accessible and suitable stream nearby (Stoecker 2002). This is an important adaptation in the often stochastic and arid regions of southern California.
During spawning, female steelhead create depressions in streambed gravels by vigorously pumping their body and tail horizontally near the streambed. The optimal water depth for steelhead spawning is approximately 14 in (36 cm) (Stoecker 2002). These depressions, or redds, are approximately 4–12 inches (10–30 cm) deep, 15-in (38-cm) in diameter, and oval in shape (Needham and Taft 1934, Shapovalov and Taft 1954). Males do not assist with redd construction, but may fight with other males to defend spawning females (Shapovalov and Taft 1954).
Although most steelhead die after spawning, adults are capable of returning to the ocean and migrating back upstream to spawn in subsequent years, unlike most other Pacific salmon. Runs may include from 10% to 30% repeat spawners, the majority of which are females (Ward and Slaney 1988, Meehan and Bjornn 1991, Behnke 1992). Repeat spawning is more common in smaller coastal streams than in large drainages requiring a lengthy migration (Meehan and Bjornn 1991). Steelhead may migrate downstream to the ocean immediately following spawning or may spend several weeks holding in pools before outmigrating (Shapovalov and Taft 1954).
Egg incubation, alevin development, and fry emergence
Hatching of eggs follows a 20- to 100-day incubation period, the length of which depends on water temperature (Shapovalov and Taft 1954, Barnhart 1991).
Juvenile freshwater rearing
Juvenile steelhead (parr) rear in freshwater before outmigrating to the ocean as smolts. Juvenile southern steelhead have extremely variable residence time due to the highly unpredictable and often stochastic environmental conditions that exist in watersheds in southern California (Stoecker and Kelley 2005). Some juvenile steelhead may never migrate, they remain in freshwater as coastal rainbow trout for their entire life cycle (Stoecker and Kelley 2005).
Steelhead may overwinter in mainstem reaches, particularly if coarse substrates in which to seek cover from high flows are available (Reedy 1995), or they may return to tributaries for the winter (Everest 1973, as cited in Dambacher 1991).
At the end of the freshwater rearing period, juvenile steelhead migrate downstream to the ocean as smolts. A length of 5.46 in (14 cm) is typically cited as the minimum size for smolting (Wagner et al. 1963, Peven et al. 1994). Evidence suggests that photoperiod is the most important environmental variable stimulating the physiological transformation from parr to smolt (Wagner 1974). During smoltification, the spots and parr marks characteristic of juvenile coloration are replaced by a silver and blue-green iridescent body color (Barnhart 1991) and physiological transformations occur that allow them to survive in salt water. Southern steelhead smolts may spend a considerable amount of time in lagoons and estuaries in order to acclimate to saltwater before outmigrating (Stoecker 2002). These lagoons and estuaries also provide a holding area where smolts can feed while waiting for adequate flow conditions to open the streams and lagoons to the ocean (sandbars build up and seal off many confluences in low flow conditions) (Stoecker 2002).
Estuarine rearing may be more important to steelhead populations in the southern half of the species' range due to greater variability in ocean conditions and paucity of high quality near-shore habitats in this portion of their range (NMFS 1996a). Estuaries may also be more important to populations spawning in smaller coastal tributaries due to the more limited availability of rearing habitat in the headwaters of smaller stream systems (McEwan and Jackson 1996). Most marine mortality of steelhead occurs soon after they enter the ocean and predation is believed to be the primary cause of this mortality (Pearcy 1992, as cited in McEwan and Jackson 1996). Because predation mortality and fish size are likely to be inversely related (Pearcy 1992, as cited in McEwan and Jackson 1996), the growth that takes place in estuaries may be very important for increasing the odds of marine survival (Pearcy 1992 [as cited in McEwan and Jackson 1996], Simenstad et al. 1982 [as cited in NMFS 1996a], Shapovalov and Taft 1954).
The majority of steelhead spend one to three years in the ocean, with smaller smolts tending to remain in salt water for a longer period than larger smolts (Chapman 1958, Behnke 1992). Steelhead staying in the ocean for two years typically weigh 7 to 10 lbs (3.15 to 4.50 kg) upon return to fresh water (Roelofs 1985). Unlike other salmonids, steelhead do not appear to form schools in the ocean. Steelhead in the southern part of the species' range appear to migrate close to the continental shelf, while more northern populations of steelhead may migrate throughout the northern Pacific Ocean (Barnhart 1991).
Adult upstream migration and spawning
During their upstream migration, adult steelhead require deep pools for resting and holding to minimize their energetic outputs (Puckett 1975, Roelofs 1983, as cited in Moyle et al. 1989, Stoecker and Kelly 2005). Southern steelhead require spawning areas to be at least 14 inches (36 cm) deep (SCREMP 1996). Steelhead need water with a minimum depth of 7 in (18 cm) and maximum velocity of 8 ft/s (240 cm/s) for successful upstream migration (Thompson 1972, as cited in Everest et al. 1985). Relatively cool water temperatures (between 50 and 59?F [10o and 15?C]) are preferred by adults, although they may survive temperatures as high as 80.6?F (27) for short periods (Moyle et al. 1989). Pool tailouts or heads of riffles with well-oxygenated gravels are often selected as redd locations (Shapovalov and Taft 1954). The average area encompassed by a redd is 47–65.56 ft2 (4.4–5.9 m2) (Orcutt et al. 1968, Hunter 1973, as cited in Bjornn and Reiser 1991). Gravels ranging in size from 0.25 to 5.07 in (0.64 to 13 cm) in diameter are suitable for redd construction (Barnhart 1991).
Egg incubation, alevin development, and fry emergence
Incubating eggs require dissolved oxygen concentrations, with optimal concentrations at or near saturation. Low dissolved oxygen increases the length of the incubation period and cause emergent fry to be smaller and weaker. Dissolved oxygen levels remaining below 2 ppm result in egg mortality (Barnhart 1991).
Juvenile freshwater rearing
Age 0+. After emergence from spawning gravels in spring or early summer, steelhead fry move to shallow-water, low-velocity habitats such as stream margins and low-gradient riffles and will forage in open areas lacking instream cover (Hartman 1965, Everest et al. 1986, Fontaine 1988). As fry increase in size in late summer and fall, they increasingly use areas with cover and show a preference for higher-velocity, deeper mid-channel waters near the thalweg (Hartman 1965, Everest and Chapman 1972, Fontaine 1988).
Age 1+ and older juveniles. Older age classes of juvenile steelhead (age 1+ and older) occupy a wide range of hydraulic conditions. They prefer deeper water during the summer and have been observed to use deep pools near the thalweg with ample cover as well as higher-velocity rapid and cascade habitats (Bisson et al. 1982, Bisson et al. 1988). Age 1+ fish typically feed in pools, especially scour and plunge pools, resting and finding escape cover in the interstices of boulders and boulder-log clusters (Fontaine 1988, Bisson et al. 1988). During summer, steelhead parr appear to prefer habitats with rocky substrates, overhead cover, and low light intensities (Hartman 1965, Facchin and Slaney 1977, Ward and Slaney 1979, Fausch 1993). Age 1+ steelhead appear to avoid secondary channel and dammed pools, glides, and low-gradient riffles with mean depths less than 7.8 in (20 cm) (Fontaine 1988, Bisson et al. 1988, Dambacher 1991).
As steelhead grow larger, they tend to prefer microhabitats with deeper water and higher velocity as locations for focal points, attempting to find areas with an optimal balance of food supply versus energy expenditure, such as velocity refuge positions associated with boulders or other large roughness elements close to swift current with high macroinvertebrate drift rates (Everest and Chapman 1972, Bisson et al. 1988, Fausch 1993). Reedy (1995) indicates that 1+ steelhead especially prefer high-velocity pool heads, where food resources are abundant, and pool tails, which provide optimal feeding conditions in summer due to lower energy expenditure requirements than the more turbulent pool heads. Fast, deep water, in addition to optimizing feeding versus energy expenditure, provides greater protection from avian and terrestrial predators (Everest and Chapman 1972).
Steelhead overwinter in pools, especially low-velocity deep pools with large rocky substrate or woody debris for cover, including backwater and dammed pools (Hartman 1965, Swales et al. 1986, Raleigh et al. 1984, Fontaine 1988). Juveniles are known to use the interstices between substrate particles as overwintering cover. Bustard and Narver (1975) typically found age 0+ steelhead using 3.9–9.7 in (10–25 cm) diameter cobble substrates in shallow, low-velocity areas near the stream margin. Everest et al. (1986) observed age 1+ steelhead using logs, rootwads, and interstices between assemblages of large boulders (39.0 in [>100 cm] diameter) surrounded by small boulder to cobble size (19.7–39.0 in [50–100 cm] diameter) materials as winter cover. Age 1+ fish typically stay within the area of the streambed that remains inundated at summer low flows, while age 0+ fish frequently overwinter beyond the summer low flow perimeter along the stream margins (Everest et al. 1986).
Little is known about steelhead use of ocean habitat. Some steelhead migrate extensively while others have short oceanic migrations (Stoecker and Kelly 2005). Steelhead appear to prefer ocean temperatures of 48.2o–52.7oF (9o–11.5oC) and typically swim in the upper 30–40 ft (9–12 m) of the ocean's surface (Barnhart 1991).
Emergent O. mykiss fry initially feed on zooplankton and other microorganisms (Barnhart 1991). Juveniles feed on a wide range of items, primarily those associated with the stream bottom such as aquatic insects, amphipods, aquatic worms, fish eggs, and occasionally smaller fish (Wydoski and Whitney 1979). Juveniles may also feed on spiders, mollusks, and fish, including smaller steelhead (Roelofs 1985). Age 0+ steelhead prefer benthic invertebrates (Johnson and Ringler 1980); larger steelhead, having larger mouths, can consume a broader range of foods (Fausch 1991). In the ocean, steelhead feed on juvenile greenling, squids, amphipods, and other organisms (Barnhart 1991).
Major predators of adult steelhead include humans, marine mammals, and large pelagic fish. Eggs may be eaten by macroinvertebrates, crayfish, and other fish. Juvenile steelhead may be preyed upon by garter snakes, piscivorous fish such as older salmonids (including steelhead), freshwater sculpins, introduced piscivorous fish (e.g., black bullhead, green sunfish, smallmouth bass, striped bass), mammals (e.g., river otter, mink), and piscivorous birds (e.g., mergansers, kingfishers, herons, ospreys, loons) (Stoecker and Kelly 2005). Juvenile steelhead have been observed feeding on emergent fry (Shapovalov and Taft 1954).
Sensitivity to Anthropogenic Watershed Disturbances An anadromous life history and changes in habitat requirements at different life stages make steelhead vulnerable to a wide range of watershed disturbances, including dams, timber harvest, road construction, recreational use, and other human-related disturbances. The relative importance of anthropogenic or natural disturbances and ocean conditions for controlling steelhead populations is uncertain.
Physical barriers to migration and movement
Dams without fish passage facilities block migration to historically available spawning and/or rearing areas, inundate spawning and rearing habitat beneath reservoirs, and alter hydrologic regimes, sediment and LWD budgets, water temperatures, nutrient cycling, and food supplies (Collins 1976). Where fish passage facilities are provided at dams, delays to upstream or downstream migration may occur, and stress, injury, or mortality may result from passage through juvenile bypass facilities. Stoecker and Kelly (2005) identified and assessed barriers to southern steelhead habitat throughout the Santa Clara River watershed. Severe barriers to steelhead passage were identified on tributaries to the Santa Clara River, including Santa Paula, Sespe, Hopper, and Piru Creeks (Stoecker and Kelly 2005). Additionally, the most significant barrier existing on the Santa Clara mainstem is the Vern Freeman Diversion Dam, which needs considerable improvement to allow unimpeded upstream and downstream migration over a wide range of flows, independent of water diversion operations, maintenance, debris blockage, or fish ladder damage (Stoecker and Kelly 2005).
Changes to hydrologic regimes
Changes to natural flow regimes may impact steelhead populations through changes to stimuli used for timing of upstream and downstream migrations, dewatering of redds, displacement of fry or juveniles, scouring of spawning gravels, and changes to the quality and quantity of habitat for different life stages. Rapid decreases in flow associated with hydroelectric project operations may cause stranding, especially of recently emerged fry because of their preference for stream margin areas of mainstem channels and because they are relatively weak swimmers (Hunter 1992). Vulnerability to stranding declines once juvenile steelhead reach lengths of 1.8 inches (45 mm) (R.W. Beck and Associates 1987). As juveniles grow, they are more likely to occupy deeper areas further from channel margins, reducing their susceptibility to stranding. Flow diversions may delay or stop adult migration if minimum water depths are not maintained (Everest et al. 1985).
Changes to sediment dynamics
Sedimentation of streams resulting from increased erosion may reduce spawning success of steelhead and the carrying capacity of juvenile rearing areas. Sedimentation due to land use activities has been recognized as a primary cause of habitat degradation for steelhead populations on the west coast (NMFS 1996a). Increased input of fine sediment resulting from natural or anthropogenic disturbance may be the principle cause of egg and alevin mortality in some areas (Shapovalov and Taft 1954). Filling of interstitial spaces with fine sediments reduces intragravel flow through redds, reducing dissolved oxygen concentrations and the rate of removal of metabolic wastes (Everest et al. 1985). Alevins that develop in oxygen-deficient gravels are smaller at emergence, placing them at a competitive disadvantage (Doudoroff and Warren 1965, as cited in Everest et al. 1985). Interstitial habitat used as cover by juvenile steelhead is also reduced if embedded in fine sediments. Bjornn et al. (1977) observed reduced juvenile steelhead abundance in Idaho streams characterized by a high degree of substrate embeddedness.
Changes to stream temperatures and water quality
Factors that result in increased stream temperatures, such as removal of riparian vegetation and changes to natural flow regimes may reduce steelhead populations both directly through increased mortality and indirectly through such factors as changes to growth rates or timing of emergence and downstream migration.
Warm water temperatures may favor competitors of juvenile steelhead, such as redside shiners (Reeves et al. 1987). Increases in water temperatures may also make juvenile anadromous salmonids more susceptible to mortality from diseases such as Flexibacter columnaris (Holt et al. 1975).
Estuary conditions may have an important influence on anadromous fish survival, since anadromous fish must pass through these areas during upstream and downstream migration and since estuarine rearing prior to ocean entry is a life history strategy used by many juvenile anadromous fish to increase marine survival (Giger 1972, Healey 1991, McMahon and Holtby 1992). Degradation of estuary habitats due to diking and filling, increased temperatures, introduction of piscivorous fish, sedimentation due to upstream impacts, and other human activities may have contributed to anadromous fish declines in California.
- How do non-native species (e.g., African clawed frogs, bullfrogs, smallmouth bass) impact steelhead in the Santa Clara River watershed?
- Would habitat restoration be beneficial for southern steelhead if barriers still exist downstream of the habitat restoration?
- Is it possible to improve habitat connectivity for southern steelhead?
- Is food availability a limiting factor for fry and juvenile steelhead success?
- Smolt utilization and survival in the estuary
- Steelhead ocean ecology
- How much straying occurs from natal streams?
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